In this study, we examined the acute toxicity, immunotoxicity, and cytotoxicity of bacterial magnetosomes (BMs). LD(50) of BMs injected into the sublingual vein of SD rats was 62.7 mg/kg. Further studies with injection of 40 mg/kg BMs showed no significant difference between BM-treated and control rats in terms of routine blood exam results, liver and kidney function tests, organ coefficients of major organs, or Stimulation Index (SI) of lymph cells with ConA and/or LPS antigens. Histological examination of major organs from 40 mg/kg BM-treated rats showed no obvious pathological changes except for increased number of vacuoles in livers, and somewhat thicker interlobular septa in lungs. BMs showed little cytotoxic effect on H22, HL60, or EMT-6 cells. Growth of all three cells was neither inhibited nor stimulated by incubation with 9 microg/ml BMs, which also had no effect on DNA content, cell size, or cell membrane integrity.